FIG. 12 is a cross section of a conventional exhaust gas recirculation control valve device (hereinafter simply "EGR valve"), FIG. 13 is a cross section along line XIII--XIII of FIG. 12, FIG. 14 is a cross section along line XIV--XIV of FIG. 12, FIG. 15 is a cut away perspective view of part of the stepper motor in FIG. 12, and FIG. 16 is a schematic of the interior of the stepper motor in FIG. 15. Further, FIG. 14 is expanded to twice the size of FIG. 13.
This EGR valve is provided with a valve main body 1 and a stepper motor 2 mounted on the upper portion of the valve main body 1.
The valve main body 1 is provided with a valve body 5 having an exhaust gas inflow passage 3 and an exhaust gas outflow passage 4, a valve seat 6 disposed between the exhaust gas inflow passage 3 and the exhaust gas outflow passage 4, a valve 7 in direct contact with the valve seat 6, a valve shaft 8, one end of which is fixed to the valve 7, a shaft bushing 9 disposed between the valve body 5 and the valve shaft 8 so as to slidably support the valve shaft 8, a spring bracket 10 fixed to the other end of the valve shaft 8, and a coil spring 11 compressed and disposed between the valve body 5 and the spring bracket 10.
The stepper motor 2 is provided with a motor case 20, a motor cap 21 fixed to the motor case 20, a rotor 28 disposed within the motor case 20 and rotatably supported by a shaft bushing 27, and a stator 32 disposed on the outside of the rotor 28 to turn the rotor 28.
The rotor 28 is provided with a shaft bushing 22 fixed to the motor cap 21, a motor shaft 23 having a shaft portion 24 supported by the shaft bushing 22 so as to slide freely along the direction of the shaft and a male thread portion 25, a female thread portion 26 screwing the male thread portion 25, and a cylindrical magnet portion 30 consisting of an array of alternating north pole magnets and south pole magnets disposed outside the female thread portion 26.
As shown in FIG. 13, the cross section of the shaft portion 24 of the motor shaft 23 is a segmented circle, so that the motor shaft 23 can only move along the direction of the shaft axis. Also, as shown in FIG. 14, protruding shaft portion positioning portions 29 are formed in the upper portion of the shaft portion 24. A pair of rotor positioning portions 31, which come into direct contact with the shaft portion positioning portions 29, are formed on the inner surface of the lower portion of the female thread portion 26. The upper positional limit of the motor shaft 23 is regulated by the rotor positioning portions 31 coming into contact with the shaft portion positioning portions 29. In other words, as the female thread portion 26 rotates, the motor shaft 23, which has a thread screwing that of the female thread portion 26, may move upwards, but once the rotor positioning portions 31 come into contact with the shaft portion positioning portions 29, the female thread portion 26 can no longer rotate, and so the motor shaft 23 cannot move upwards any further (the motor shaft 23 cannot rotate; it can only move along the direction of the shaft axis because of the shaft bushing 22).
The aforementioned stator 32 is provided with an upper coil 41, a lower coil 42 disposed below the upper coil 41, a first phase stator portion 43 mounted on the upper surface of the upper coil 41, a second phase stator portion 44 mounted on the lower surface of the upper coil 41, a third phase stator portion 45 mounted on the upper surface of the lower coil 42, and a fourth phase stator portion 46 mounted on the lower surface of the lower coil 42. The shape of each of the phase stator portions 43, 44, 45, 46 is annular, and each has claw portions 43a, 44a, 45a, 46a formed on its inner edge and bent towards coils 41 and 42, respectively. The claw portions 43a of the first phase stator portion 43 are arranged so as to interlock with the claw portions 44a of the second phase stator portion 44, and the claw portions 45a of the third phase stator portion 45 are arranged so as to interlock with the claw portions 46a of the fourth phase stator portion 46.
In the above EGR valve, when a current is passed through the upper coil 41 and the lower coil 42, magnetic poles is formed in each phase of the stator portions 43, 44, 45, 46 and like magnetic poles is formed in the corresponding claw portions 43a, 44a, 45a, 46a.
The direction of the current in the upper coil 41 can be reversed, and similarly the direction of the current in the lower coil 42 can also be reversed, so that there are four possible patterns of current direction and the magnetic poles which arise in each of the phase stator portions 43, 44, 45, 46 change with each pattern. Then, within the magnetic field generated by the claw portions 43a, 44a, 45a, 46a, the magnet portion 30 and the female thread portion 26 rotate to and are maintained in a position where the magnetic forces acting on between the claw portions 43a, 44a, 45a, 46a and the magnet portion 30 are in equilibrium.
Also, if the order of the above changes in current pattern (steps) is reversed, the magnetic portion 30 and the female thread portion 26 will rotate in the opposite direction.
With the rotation of the magnetic portion 30 and the female thread portion 26, the male thread portion 25, whose thread matches that of the female thread portion 26, also rotates and motor shaft 23 moves along the direction of the shaft axis.
In the aforementioned EGR valve, when the motor shaft 23 is moved downwards by the action of the aforementioned stepper motor, the motor shaft 23 starts to act midway in opposition to the elasticity of the compressed coil spring 11, pushing the head of the valve shaft 8 and moving the valve shaft 8 downwards and thus separating the valve 7 from the valve seat 6, whereby the exhaust gas inflow passage 3 connects with the exhaust gas outflow passage 4 and exhaust gas flows from the exhaust gas inflow passage 3 into the exhaust gas outflow passage 4.
By reversing the direction of rotation of the magnetic portion 30 and the female thread portion 26, the motor shaft 23 will move upwards along the direction of the shaft axis and the valve shaft 8 will also be moved upwards by the elasticity of the compressed coil spring 11, its head in contact with the shaft portion 24. Then, the valve 7 will come into contact with the valve seat 6, closing the valve main body 1, whereby the exhaust gas inflow passage 3 is cut off from the exhaust gas outflow passage 4 and exhaust gas cannot flow. If the magnetic portion 30 and the female thread portion 26 are rotated further in this direction, the motor shaft 23 will move further upwards and the shaft portion 24 will separate from the valve shaft 8.
FIG. 17 shows the relationship between the number of steps (number of changes in current pattern) in the stepper motor 2 and the amount of flow through the EGR valve. It can be seen from the graph that the amount of flow is proportional to the number of steps.
Now, in order to operate the stepper motor 2 exactly as instructed by the control unit (not shown), it is necessary to initialize the position of the motor shaft 23 of the stepper motor 2 beforehand.
To perform this initialization reliably, the stepper motor 2 is given a greater number of steps than is needed to place the motor shaft 23 of the stepper motor 2 at the end of the motor. In this way, the shaft portion 24 of the motor shaft 23 is separated to an appointed distance from the head of the valve shaft 8, and once the motor shaft 23 reaches the motor end position, even if current is passed through the upper coil 41 and lower coil 42, generating magnetic poles in each of the phase stator portions 43, 44, 45, 46 and rotating the magnetic portion 30 and the female thread portion 26 in an attempt to move the motor shaft 23 upwards, the rotor positioning portions 31 will come into contact with the shaft portion positioning portions 29 of the shaft portion 24 and the magnetic portion 30 and the female thread portion 26 will not be able to rotate further, and so, once the motor shaft 23 reaches the motor end position, it does not move any further into the female thread portion 26.
Next, the motion of the magnet portion 30 after the motor shaft 23 has reached the motor end shall be explained on the basis of FIG. 18.
FIGS. 18(a) to (d) show the changes in the magnetization of each of the stator portions 43, 44, 45, 46 when the motor shaft 23 is moved into the female thread portion 26. The magnetization of each of the stator portions 43, 44, 45, 46 changes in the order of FIG. 18(a), FIG. 18(b), FIG. 18(c), and FIG. 18(d), and after FIG. 18(d), it returns to that of FIG. 18(a).
FIG. 18(a) shows the magnetization of each of the stator portions 43, 44, 45, 46 and the position of the magnet portion 30 at the instant the motor shaft 23 reaches the motor end.
FIG. 18(b) shows the magnetization of each of the stator portions 43, 44, 45, 46 intended to apply a single step of rotational force to the magnet portion 30 in the direction of the arrow A. The force acts on the magnet portion 30 in the direction of the arrow A, but the rotor positioning portions 31 come into contact with the shaft portion positioning portions 29 and the magnet portion 30 cannot rotate.
FIG. 18(c) shows the magnetization of each of the stator portions 43, 44, 45, 46 intended to apply a further single step of rotational force to the magnet portion 30 in the direction of the arrow A. The force acts further on the magnet portion 30 in the direction of A, but the rotor positioning portions 31 come into contact with the shaft portion positioning portions 29 and the magnet portion 30 cannot rotate.
FIG. 18(d) shows the magnetization of each of the stator portions 43, 44, 45, 46 intended to apply a further single step of rotational force to the magnet portion 30 in the direction of the arrow A. However, from this position, the magnetic forces acting between the stator portions 43, 44, 45, 46 and the magnet portion 30 rotate the magnet portion 30 and stabilize one step in the direction of arrow B.
When the stepper motor is again magnetized to turn the rotor 28 one step in the direction of the arrow A, the magnetization of each of the stator portions 43, 44, 45, 46 and the position of magnet portion 30 is as shown in FIG. 18(a), and the magnet portion 30 moves in the direction to collide with the shaft portion positioning portions 29. Thereafter, the above operation is repeated as part of the process of initialization, and on completion of initialization, the magnetization of the stator portions 43, 44, 45, 46 is as shown in FIG. 18(a) and the motor shaft 23 is positioned at the motor end.
The conventional EGR valve is constructed as described above and during the process of initialization, when the magnet portion 30 moves from the position shown in FIG. 18(d) to that shown in FIG. 18(a), the rotor positioning portions 31 of the female thread portion 26, which move synchronously with the magnet portion 30, collide with the shaft portion positioning portions 29 and generate sound.
In particular, because the initialization of the stepper motor 2 is performed at quiet times immediately after the internal combustion engine has been switched off or just before the engine is started again, the noise of the impact can be heard by the driver in the car.
The present invention aims to solve such problems and its objective is to provide a control device to reduce the volume of the impact noise generated when the rotor positioning portions collide with the shaft portion positioning portions during the process of initialization.